The objective of the research contained in this application is to elucidate how the structural features of carotenoids determine their photochemical properties in vivo. Particular attention will be given to the roles of carotenoids in light-harvesting and photoprotection which are essential for the survival of photosynthetic organisms. The systems to be studied are carotenoids in vitro and contained in several different photosynthetic reaction center and antenna complexes that have been isolated, purified, pigment-reconstituted, pigment exchanged, crystallized or genetically-altered. X-ray crystallography, nuclear magnetic resonance, and resonance Raman spectroscopy will be used as direct structural probes. Absorption, fluorescence, fluorescence excitation, fast-transient absorption, two-photon spectroscopy, linear dichroism, electron paramagnetic resonance spectroscopy, and semi-empirical MO calculations will allow the correlation of structure with the spectroscopic observables and function. The data will provide a basis from which to understand more complicated organisms; e.g. higher plant pigment-protein complexes. The specific aims of this proposal are: (1) To examine the structures of carotenoids in photosynthetic pigment-protein complexes; (2) To explore the relationships between the carotenoid structures and their spectroscopic properties; and (3) To determine the relationships between the carotenoid structures and the mechanisms by which they function in vivo.